In recent years, many more users demand fast data transmission in a radio communication system as the volume of communication increases. The multicarrier transmission represented by OFDM (Orthogonal Frequency Division Multiplexing) gets attention as a way of communication to realize the fast and high-volume data transmission. The OFDM, which is used in IEEE 802.11a being a radio system of a 5 GHz-band or digital terrestrial broadcast, provides for simultaneous communication by arranging tens to thousands of carriers in a minimum frequency interval that does not induce interference theoretically. Generally in OFDM, these carriers are referred to as subcarriers which are digitally modulated with PSK, QAM or the like for communication. It is known that OFDM and error correction are combined to obtain strong tolerance to frequency selective fading.
The patent literature 1 discloses a configuration example of a receiving device used in a communication system using the above OFDM. FIG. 13 is a diagram showing a device configuration example of a receiver illustrated in the patent literature 1. As shown in FIG. 13, the receiver proposed in the patent literature 1 is configured to include an antenna unit 1000, a GI (Guard Interval) removing unit 1001, an FFT unit 1002, demodulating units 1003-1 to -N, a P/S (Parallel to Serial) converting unit 1004, a selecting unit 1005, switches 1006-1 to -N, an IFFT unit 1007, a delay profile estimating unit 1008 and an FFT unit 1009.
A guard interval is first removed in the GI removing unit 1001 from a received signal that has been received in the antenna unit 1000 of the receiver shown in FIG. 13, and then the signal is converted from a time domain signal into a frequency domain signal in the FFT unit 1002. An information signal of each subcarrier obtained in the FFT unit 1002 as described above is transmitted to the demodulating units 1003-1 to -N for demodulation. Generally in a broadcasting system, a wireless LAN system and other systems, a known pilot signal to estimate a channel response is contained in a symbol or in a packet in addition to an information signal. Such a pilot signal is selected in the selecting unit 1005 and transmitted to the IFFT unit 1007 via the switches 1006-1 to -N.
The pilot signal transmitted to the IFFT unit 1007 is converted from a frequency domain signal into a time domain signal and a delay profile of a channel response is estimated in the delay profile estimating unit 1008. Then, the delay profile of the channel response estimated from the known pilot signal as described above is converted from a time domain signal into a frequency domain signal in the FFT unit 1009. The processing enables to seek a channel response in frequency domain.
The channel response in frequency domain obtained by the above processing is transmitted to the demodulating unit 1003-1 to -N to which an information signal has been transmitted, for use in compensation of the channel response, thereby enabling demodulation by compensating for influence of multipath fading and accordingly enabling to correctly demodulate an information signal.    [Patent Literature 1] JP Patent Publication (Kokai) No. 2003-32217 (2003)
The above prior art enables to estimate variance of a channel response for an information signal and compensate for influence of the variance. However, the art has a problem in that influence of interference waves coming from adjacent cells significantly degrades the accuracy of the estimation processing of a channel response in the circumstance of another OFDM system nearby using the same frequency band, or in the circumstance of a terminal being around a cell edge in an OFDM cellular system in which all cells use the same frequency band; as a result, an information signal cannot be demodulated correctly.
The art has another problem in that if a base station specific code is used in an OFDM symbol for estimating a channel response, to identify the base station in a cellular system, the base station specific code needs to be detected to estimate the channel response. This is problematic in that a usual correlation method in a frequency domain does not correctly work in the environment with strong frequency selective fading.
Similarly, in the case that a transmitting device implements a transmit diversity technique using a plurality of transmitting antennas, codes to estimate channel responses use the same code. That is, a technique has not been disclosed for both estimating channel responses and identifying antennas, using the same OFDM symbol.
It is an object of the present invention to precisely estimate a channel response and correctly demodulate an information signal even in the above circumstances.